The present invention is in the field of polymer foams. Specifically, the present invention relates to polymer nanocomposite foams.
The present invention hereby incorporates by reference, application No. 10/425,565, entitled xe2x80x9cClay Nanocomposites Prepared by In-situ Polymerizationxe2x80x9d, filed on Apr. 29, 2002.
Foamed polymers are found in applications ranging from packaging, insulation, cushions, adsorbents, to scaffolds for tissue engineering. The basic principle of foaming is to mix a blowing agent (typically a gas) into a polymer melt and induce a thermodynamic instability through a temperature or pressure change to nucleate bubbles of the blowing agent.
In this invention, supercritical CO2 (the critical temperature Tc: 31xc2x0 C. and the critical pressure Pc: 73.8 bar or 1074 psi), a potential replacement of the traditional foaming agents (hydrocarbon or chlorofluorocarbon), was applied, The liquid-like solubility and gas-like diffusivity make it possible to dissolve sufficient CO2 in a polymer quickly. CO2 is low-cost, non-flammable, chemically benign, and environmentally friendly.
Recently, microcellular foams, characterized by cell sizes smaller than 10 xcexcm and cell density larger than 109 cells/cm3, have drawn a great deal of attention and interest. It has been shown that by keeping the cell (or bubble) size uniformly less than 10 microns in diameter, one can greatly reduce material usage without compromising mechanical properties because the bubbles are smaller than the preexisting flaws in a polymer matrix.
The field of polymer/clay nanocomposites has grown rapidly in the past decade. In this work, nano-sized particles, nanoclays, are applied to modify the cellular foams in both batch and continuous extrusion foaming process. The results show that with the addition of a very small amount of nanoclay into the polymer matrix, the nanocomposites exhibit substantial increase in many physical properties, including mechanical strength (tensile modulus and strength, flexural modulus and strength), thermal stability, flame retardance, and barrier resistance. Smectite clays, such as montmorillonite (MMT), are of particular interest because they have a high aspect ratio (lateral dimension xcx9c200-500 nm, thickness  less than 1 nm) and a high surface area. However, clay is hydrophilic in nature and incompatible with most polymers. To increase the compatibility and miscibility of clay in polymer, the clay surface is modified by an organic surfactant, typically ammonium cations with long alkyl chains.
Two idealized polymer/clay structures are possible: intercalated and exfoliated. Exfoliation involves extensive polymer penetration to disrupt the clay crystallite (tactoids), and the individual nanometer-thick silicate platelets are dispersed in the polymer matrix. If there is only limited polymer chain insertion in the interlayer region, and the interlayer spacing only expands to a certain extent without losing layer registry, then an intercalated nanocomposites is then formed.
Polymer foam is another area subject to intensive research. It is widely used for insulation, packaging, and structural applications, to name a few. Microcelluar foam, which is characterized by cell size in the range of 0.1xcx9c10 xcexcm, cell density in the range of 109 to 1015 cells/cc, provides improved mechanical properties as well as increased thermal stability and lower thermal conductivity.
Cell nucleation and growth are two important factors controlling cell morphology. Particles can serve as a nucleation agent to improve heterogeneous nucleation. Some inorganic nucleation agents, such as talc, silicon oxide, kaoline, etc., are widely used. A fine dispersion of these nucleation agents can promote formation of nucleation center for the gaseous phase. Although a detailed explanation of the heterogeneous nucleation mechanism is still not available, the size, shape, and distribution, and surface treatment of particles have great influences on the nucleation efficiency. In this work, we developed a new polymer nanocomposite foam preparation technology to create polymer foams with controlled cell structure. In addition, clay may further improve the foam properties, e.g., mechanical and barrier properties, as well as fire resistance.
The present invention includes polymeric nanocomposite foams and a method for forming polymeric nanocomposite foams.
A method for forming a polymeric nanocomposite foam of the present invention comprises the steps of providing a mixture comprising: a polymer, an organophilic clay, and a blowing agent; and processing said mixture so as to cause formation of cells, thereby forming a polymeric nanocomposite foam.
Although any appropriate amount of blowing agent may be used, it is preferred that the mixture comprises at least 1% by weight of the blowing agent. It is more preferred that the mixture comprise at least 4% by weight of the blowing agent. It is most preferred that the mixture comprises at least 7% by weight of said blowing agent.
Although any desired amount of organophilic clay may be used, it is preferred that the mixture contain at least 0.5% by weight of the organophilic clay. It is more preferred that the mixture comprise at least 5% by weight of the organophilic clay. It is further preferred that the mixture comprise at least 10% by weight of the organophilic clay. It is most preferred that the mixture comprises at least 20% by weight of the organophilic clay.
While any appropriate polymer may be used in forming the polymeric nanocomposite foam, it is preferred that the polymer is selected from the group consisting of polystyrene, poly(methyl methacrylate), polypropylene, nylon, polyurethane, elastomers, and mixtures thereof.
It is preferred that the organophilic clay is dispersed throughout the polymer such that a x-ray diffraction pattern produced from the mixture is substantially devoid of an intercalation peak for producing exfoliated polymeric nanocomposite foams. It is preferred that organophilic clay is dispersed throughout the polymer such that a x-ray diffraction pattern produced from the mixture contains an intercalation peak for producing intercalated polymeric nanocomposite foams.
It is preferred that the organophilic clay comprises: a smectite clay; and a compound having the formula: 
wherein R1 is (CH)n wherein n ranges from 6 to 20; R2 is a chemical structure having a terminal reactive double bond; R3 is an alkyl group; and R4 is an alkyl group.
It is most preferred that the compound have n=15, R3 as CH3, R4 as CH3, and R2 as: 
While any appropriate clay may be used, it is preferred to use smectite clay. It is more preferred that the smectite clay is selected from the group consisting of montmorillonite, hectorite, saponite, laponite, florohectorite, and beidellite.
The blowing gas may be any traditional blowing gas used in industry (for example: freon, nitrogen or air). However, it is preferred that the blowing agent is a supercritical fluid. It is most preferred that the blowing agent is supercritical carbon dioxide.
Cell size can vary widely depending upon operating conditions, however, it is preferred that the polymeric nanocomposite foam has an average cell size less than about 20 microns. It is additionally preferred that the polymeric nanocomposite foam has an average cell size greater than about 15 microns.
Cell density can vary widely depending on operating conditions, however, it is preferred that the polymeric nanocomposite foam has an average cell density greater than about 1xc3x97106 cells/cm3. It is more preferred that the polymeric nanocomposite foam have an average cell density greater than about 1xc3x97109 cells/cm3.
The polymer nanocomposite foam may be closed cell foam or open cell foam.
A polymeric nanocomposite foam of the present invention comprises a polymeric portion; an organophilic clay, the organophilic clay is dispersed throughout the polymeric portion; and a plurality of cells dispersed throughout the polymeric portion.
While any appropriate polymer may be used in the polymeric nanocomposite foam, it is preferred that the polymeric portion comprises a polymer selected from the group consisting of polystyrene, poly(methyl methacrylate), polypropylene, nylon, polyurethane, elastomers, and mixtures thereof.
It is preferred that the organophilic clay is dispersed throughout the polymer such that a x-ray diffraction pattern produced from the mixture is substantially devoid of an intercalation peak for exfoliated polymeric nanocomposite foams. It is preferred that organophilic clay is dispersed throughout the polymer such that a x-ray diffraction pattern produced from the mixture contains an intercalation peak for intercalated polymeric nanocomposite foams.
While any organophilic clay may be used, it is preferred that the organophilic clay portion comprises: a smectite clay; and a compound having the formula: 
wherein R1 is (CH)n wherein n ranges from 6 to 20; R2 is a chemical structure having a terminal reactive double bond; R3 is an alkyl group; and R4 is an alkyl group.
It is most preferred that the compound have n=15, R3 as CH3, R4 as CH3, and R2 as: 
While any appropriate clay may be used, it is preferred to use smectite clay. It is more preferred that the smectite clay is selected from the group consisting of montmorillonite, hectorite, saponite, laponite, florohectorite, and beidellite.
Cell size can vary widely depending upon operating conditions, however, it is preferred that the polymeric nanocomposite foam has an average cell size less than about 20 microns. It is additionally preferred that the polymeric nanocomposite foam has an average cell size greater than about 15 microns.
Cell density can vary widely depending on operating conditions, however, it is preferred that the polymeric nanocomposite foam has an average cell density greater than about 1xc3x97106 cells/cm3. It is more preferred that the polymeric nanocomposite foam has an average cell density greater than about 1xc3x97109 cells/cm3.
The polymer nanocomposite foam may be closed cell foam or open cell foam.